1. Field of the Invention
The present invention relates in general to air bag passive restraint systems. In particular, the present invention relates to improved seals for a gas filter within, improved filter structures for, and an improved port arrangement for, a pyrotechnic air bag inflator.
2. Description of the Related Art
Air bag systems typically include a housing which contains a pyrotechnic igniting charge and a pyrotechnic gas generating material, each in a separate section or chamber. The housing includes numerous internal ports for combustion gasses of the igniting charge to contact the gas generating material, causing a reaction to produce a larger quantity of inflation gas. The housing also includes numerous exit ports for allowing the generated inflation gas to pass into an inflatable cushion. Because the gas generating material produces high temperature gas containing high temperature particulates, it is common to filter the gas before it passes through the housing ports.
For driver side air bags, the air bag housing is typically located in the steering wheel, and is typically formed as a short cylinder, with the longitudinal axis of the cylinder being parallel to the rotation axis of the steering wheel. Centrally located within this cylinder is the igniting charge and an initiator or squib. Circumferentially surrounding (and possibly underlying or overlying) the initiating charge is the mass of pyrotechnic gas generating material. Such material is typically granulated or pelletized in nature, and may have various chemical compositions (e.g., azide).
With this basic design there are two common locations for the exit ports: axial or radial. For axial designs, the ports are located on the upper longitudinal end of the cylinder, and the filter medium is a disk or annulus located just below the ports. The gas generating medium is thus held as a short but relatively thick tube. For radial designs, the ports are located on the circumferential face of the cylinder, and the filter medium is a short tube located radially inward of the ports. In this arrangement the gas generating material is held in a taller and thinner tube configuration.
For both port arrangements it is desired first that the gas generating material be capable of quickly generating the gas. For this purpose the axial design has inherent advantages. Specifically, the combustion products from the ignitor will jet radially from the interior ports to contact the gas generating material. It can be envisioned that these radial jets would impact more upon a short thick tube than a tall thin tube of gas generant. This is because the short tube has more material in the radial direction in the jet path, and less material at the longitudinal ends, out of the jet path. As such, it is more difficult to achieve the proper gas generation with radial designs.
A second desire in air bag design is that the gas pass through the filter medium to cool the gas, reduce its velocity, and to remove particulates. This is best achieved when the gas passes evenly through the entire volume of the filter. This even flow is difficult to achieve, however.
One obstacle to even flow is the placement of the filter against the upper (for axial) or outer (fore radial) wall of the housing for support against the expansive pressure. These outer walls of course include the discrete spaced exit ports. The flow through the filter is thus greater in the areas over the ports than in the areas over the space between the ports, as the pressure differential across the filter is much greater over the ports than over the solid wall.
A further obstacle to even flow from one filter face to the other is that the gas may find a path of less resistance. For example, the gas may pass into an edges of the filter (i.e., the circumferential edge of the disk or annulus, and the longitudinal ends of the tube) and out the rear face, since this path can provide reduced resistance. At the most extreme, the gas may simply pass around the ends of the filter entirely. To avoid both situations, the filter is often sealed within the housing.
Specifically, the disk filter is compressed against the upper longitudinal end by the presence of the gas generant, or a tubular prefilter. For tubular filters, the height of the filter is set slightly larger than the interior height of the housing, forcing the filter ends against the housing interior. These arrangements of course require that the respective sizes of the housing interior, filter and generant volume be tightly controlled to ensure that the filter is placed in the proper amount of compression. Furthermore, these tolerances must be even tighter when the filter is not of the typical screens and papers, but is instead formed of a single rigid mass. To reduce the need for very close tolerances, the prior art has often used a separate, resilient cushion/seal component.
Finally, in prior art arrangements which use such separate seals, the material forming the seal must be chosen carefully. The material must provide a sufficient seal, of course. However, the material must also withstand the high temperatures and pressure achieved during gas generation. Additionally, the sealed nature of the housing requires that the seal material be subject to very little outgassing.